The present application claims priority to Japanese Application(s) No(s). P2000-338288 filed Sep. 29, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to high purity cobalt in which a content of impurities such as copper is reduced, a method of manufacturing thereof, and high purity cobalt targets.
2. Description of the Related Art
Semiconductor devices such as VLSI (very large scale integrated circuit) and ULSI (ultra LSI), have a structure where various thin metal films are deposited on, for example, a silicon (Si) wafer. Although the idea of using cobalt (Co) as a wiring material has been considered in recent years, the accompanying injurious impurities in the cobalt may result in malfunction or deterioration of the semiconductor device, which is undesirable. For example, copper (Cu) may cause a short circuit because of high diffusion rate inside silicon, and radioactive elements such as uranium (U) and thorium (Th) will cause incorrect operations, and alkaline metals and alkaline-earth metals may cause degradation of the device properties.
While levels in purity of the crude cobalt traded globally and presently are about 98% to 99.8%, such crude cobalt contains various impurities, for example, transition metals such as nickel (Ni) iron (Fe), and chromium (Cr), gas elements such as oxygen (O), nitrogen (N), and sulfur (S). Therefore, in order to use cobalt for a semiconductor device, it is necessary to remove these impurities from the crude cobalt and achieve higher purification. Moreover, cobalt appears favorable as material of devices such as magnetic recording mediums or magnetic recording heads, as well as semiconductor devices, because of bearing properties typical of ferromagnetic metals. A higher purification of cobalt is indispensable to the use of cobalt as material of these devices.
Various methods of removing impurities from crude cobalt, for example, wet processing such as solvent extraction, ion exchange, and electrolytic refining for separation of metallic elements, and dryness hydrogen gas (H2) processing for removal of gas elements such as oxygen and nitrogen, and floating zone melting refining method, have been studied.
However, there is a problem with the solvent extraction. It is difficult to control extraction and reverse extraction and to refine cobalt surely in industrial processes. And, although nearly all of metal impurities can be separated by the ion exchange, copper contents before and after refining by the ion exchange may not change, that is, it is difficult to remove copper, which is a problem with the ion exchange. In addition, there are problems with the electrolytic refining that pH control of electrolytic solutions is required, and it is difficult to remove nickel and copper. And, although the floating zone melting refining method is intended to further raise the purity level of metals purified to some extent, it is reported that the floating zone melting refining method has very few effects on purification of cobalt (V. G. Glebovsky, et al. Materials Letters, 36 (1998), pp.308-314). Therefore, a need exists for methods of purifying cobalt surely, easily, and highly, and particularly for the development of methods of removing copper.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide high purity cobalt and high purity cobalt targets in which contents of impurities such as copper are reduced.
It is another object of the invention to provide a method of easily and surely manufacturing high purity cobalt.
The invention provides high purity cobalt with 99.99 mass % or more in purity wherein a copper impurity content is 50 mass ppb or less.
In another aspect, the invention provides high purity cobalt wherein a residual resistivity ratio thereof is 150 or more, and a copper impurity content is 50 mass ppb or less.
A method of manufacturing high purity cobalt according to the invention includes the steps of; converting divalent copper ions as impurities contained in an aqueous solution of cobalt chloride to monovalent copper ions; adjusting a concentration of hydrochloric acid in a range of 0.1 kmol/m3 to 3 kmol/m3; and separating the monovalent copper ions from the aqueous solution of cobalt chloride by using anion exchange resins.
In another aspect, a method of manufacturing high purity cobalt according to the invention includes the step of heating cobalt chloride or hydrates thereof from 623K to less than 873K in a hydrogen atmosphere in order to obtain cobalt.
The invention provides high purity cobalt targets with 99.99 mass % or more in purity wherein a copper impurity content is 50 mass ppb or less.
In another aspect, the invention provides high purity cobalt targets wherein a residual resistivity ratio is 150 or more, and a copper impurity content is 50 mass ppb or less.
In the high purity cobalt and the high purity cobalt targets according to the invention, concentration of copper is reduced to 50 mass ppb or less to achieve high purification.
The method of manufacturing the high purity cobalt according to the invention includes the steps of converting divalent copper ions to monovalent copper ions, and adjusting a concentration of hydrochloric acid. These steps allow copper to be absorbed on anion exchange resins, and cobalt not to be absorbed thereon. Thus the copper can be separated easily and surely from the aqueous solution of cobalt chloride.
Another method of manufacturing high purity cobalt according to the invention includes the step of heating cobalt chloride or hydrates thereof from 623K to less than 873K in a hydrogen atmosphere in order to obtain high purity cobalt easily and surely.
Other and further objects, features and advantages of the invention will appear more fully from the following description.